Various types of fluoroelastomers are known in the art, which are widely used in all those fields where optimal elastic properties associated with high chemical and thermal stability are required. For a broad treatment of such products see for instance "Ullmann's Encyclopedia of Industrial Chemistry", vol. A-11, pag. 417-429 (1988, VCH Verlagsgesellschaft). They are copolymers based on vinylidene fluoride (VDF) and/or on tetrafluoroethylene (TFE), wherein such monomers are copolymerized with other fluorinated ethylenically unsaturated monomers, such as hexafluoropropene (HFP), perfluoroalkylvinylethers (PAVE), chlorotrifluoroethylene (CTFE) and the like, or also with non fluorinated monomers having ethylene unsaturation, such as ethylene and propylene.
The fluoroelastomers can be ionically cured by addition of suitable curing agents (for instance polyhydroxylic compounds, such as Bisphenol AF or Bisphenol A), of accelerators (for instance ammonium, phosphonium, or aminophosphonium salts), and of bivalent metals oxides and/or hydroxides (for instance MgO, Ca(OH).sub.2).
Peroxide-curable fluoroelastomers have been developed more recently, which contain iodine and/or bromine atoms along the polymeric chain and/or in terminal position. Such iodine and/or bromine atoms can be introduced by suitable iodinated and/or brominated comonomers, or by using during the polymer preparation iodinated and/or brominated chain transfer agents (see for instance U.S. Pat. Nos. 4,243,770, 4,501,869 and 4,745,165). In the presence of free radicals, coming for instance from a peroxide, the iodine and/or bromine atoms act as cure-sites owing to the homolytic scission of the carbon-halogen bonds.
Since the homolytic scission of C--I bonds is much faster than that of C--Br bonds, the fluoroelastomers containing iodine show much higher curing rates than those of the brominated products. On the other hand the greatest transience of the C--I bonds involves problems in the use and in the conservation of the iodinated fluoroealastomers, mainly due to their poor stability to light. Moreover, it is known that toxic volatile by products, in particular CH.sub.3 I, are generated during curing, whose cancerogenous potential is notoriously much higher than that of CH.sub.3 Br.